It is well known that the Ti--Al intermetallic compound has fairly mach the maximum high temperature specific strength of metallic materials, and further, is high in corrosion resistance and light in weight. Metallurgical Transaction, Vol. 6A, p.1991 (1975) reported that a hightemperature strength of 40 kg/mm.sup.2 was obtained at 800.degree. C. Therefore, it has been considered optimal to use these characteristics and apply the Ti--Al intermetallic compound to gas turbine components, valves and pistons of automobile engines and apply them to dies used at high temperature, bearing parts, etc.
The Ti--Al intermetallic compound has a composition range in a phase diagram, and at a Ti content of 40 to 52 atomic percent and an Al content of 60 to 48 atomic percent in a heat equilibrium state becomes a single phase of an L1.sub.0 structure (basically, a face-centered tetragonal structure, but layers of Ti and Al are arranged intersectingly in the &lt;001&gt; direction). It has been found that an abnormal strengthening phenomenon occurs whereby the strength of the Ti--Al intermetallic compound in a single crystal state increase with an elevation of the temperature. It is known that the strength of the Ti--Al intermetallic compound in a polycrystalline state is not lowered under a high temperature, but the polycrystalline Ti--Al intermetallic compound has disadvantages of a low ductility in the temperature range of room temperature to about 700.degree. C. (Japanese Examined Patent Publication No. Sho 59-581), and the hot rolling of the polycrstalline Ti--Al intermetallic compound is very difficult. Accordingly, near-net-shape casting techniques which gives close to the final product must be employed to produce Ti--Al intermetallic compound sheets.
Recently, rapid progress has been made in near-net-shape casting techniques and, particularly when processing metallic materials, have been progressively applied to producing stainless steel sheet etc. Various casting methods as sheet manufacturing techniques have been proposed, and among those previously proposed casting methods, it has been found that a twin-drum method is suitable for producing a continuous sheet having a uniform thickness.
As an exemplary application of the foregoing techniques to an intermetallic compounds, there is known the example of a Ni--Al intermetallic compound (Ni.sub.3 Al) having an improved ductility by adding a small amount of boron. This example is reported in the international conference being held in November 1988, on "Casting of Near-Net-Shape Products" (the proceedings of an International Symposium on Casting of Near Net Shape Products, pp.315-333, issued by The Metallurgical Society.). A Ti--Al intermetallic compound sheet producing method is also disclose in Japanese Patent Application No. Hei 1-50649.
Although the application of a direct sheet process to the obtaining of near-net-shape products has the advantage of a curtailment of the manufacturing processes, a rapid cooling of the cast sheet in the direct sheet manufacturing process produces defects, such as surface cracks and porosities, in the sheet.
Accordingly, it is important to eliminate those defects in sheets produced by direct casting, to ensure sound and highly reliable sheet products.